Apparently UDFHour.java does have two evaluate() functions. One that does accept a Text object as parameter and one that does use a TimeStampWritable object as parameter. The first function does return the value of Calendar.HOUR_OF_DAY and the second one of Calendar.HOUR. In the documentation I couldn't find any information on the overload of the evaluation function. I did spent quite some time finding out why my statement didn't return a 24 hour clock value.
Shouldn't both functions return the same?